Summary of Work: Genetic and Environmental Sources of Personality Structure For decades personality psychologists worked to define the phenotypic structure of personality--the pattern of covariation of observed personality traits. The present near consensus on the Five-Factor Model (FFM) has allowed researchers to proceed to a new question: What accounts for the observed structure? Although individual personality traits are known to be strongly heritable, the covariation among different traits and the phenotypic structure of personality may or may not show genetic influences. For example, it would make little sense to look for the genes responsible for Extraversion if Extraversion were merely a cluster of genetically unrelated traits inculcated by certain patterns of child rearing. By using the intercorrelations of traits from twins, we can seek to determine whether the covariation has a common genetic origin, or whether the structure of genetic influences is similar to observed phenotypic structures. Non-shared environmental effects are usually estimated as a residual term that may also include systematic bias, such as that introduced by implicit personality theory. To reduce that bias, we supplemented data from Canadian and German twin studies with cross-observer correlations on the Revised NEO Personality Inventory. The hypothesized five-factor structure was found in both the phenotypic and genetic/familial covariances. When the residual covariance was decomposed into true non-shared environmental influences and method bias, only the latter showed the five-factor structure. True non-shared environmental influences are not structured as genetic influences are, although there Genetic covariance analyses of twin data from Canada and Germany suggested that the genotypic structure of personality mirrors the phenotypic structure. Using cross-observer data from the Baltimore Longitudinal Study of Aging to control for biases in self-report methods, analyses suggested that environmental influences on trait covariance are very limited. The present analyses show that such traits as imagination, aesthetics, sensitivity, need for variety, intellectual curiosity and liberal thinking share a common set of genetic influences that unite them into a single factor of Openness to Experience. Similarly, the other dimensions of the Five-Factor Model are determined by genetic influences. These data reaffirm the value of behavior genetic analyses for research on the underlying causes of personality traits.